This question is an ongoing one here at the offices of the American Fastener Journal. We get this question or how big is the fastener industry at least once a week. With the introduction of AmazonSupply.com the question came up again this past week.
Following is our list of distributors which handle fasteners to a large part, but may also be considered an industrial distributor. They are in Alpha Order and not by sales size:
Copper State Bolt & Nut
Rule of Thumb: It takes in excess of $100M in annual fastener sales to make the list.